Electroconductive particles and method for adjusting the isoelectric point thereof

ABSTRACT

An electroconductive powder composition of antimony-containing tin oxide with an outer layer of hydrous metal oxide having an isoelectric point in the range from about 5 to 9.

This application is a continuation-in-part of application Ser. No.07/388,921 filed Aug. 3, 1989, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to an improved electroconductive powdercomposition comprising powder particles which are generally smaller than1000 microns, and frequently tens of microns to sub-micron in size eachhaving a thin conductive coating layer which comprises a network ofinterconnecting crystallites of antimony-containing tin with an outerlayer of a hydrous metal oxide, and, more particularly, the presentinvention relates to a method for adjusting the isoelectric point ofsuch particles without substantially reducing the electroconductivitythereof by applying thereto a thin outer coating of a high isoelectricpoint hydrous metal oxide.

U.S. Pat. Nos. 4,373,013 and 4,452,830 describe an electroconductivepowder in which a substrate of titanium dioxide particles are coatedwith a conducting layer of antimony-containing tin oxide. The powder isprepared by mixing an aqueous dispersion of titanium oxide particleswith a solution containing a hydrolyzable tin salt and a hydrolyzableantimony salt. The coated titanium dioxide particles are recovered byfiltration and then heated to improve crystallinity.

Copending U.S. patent application Ser. No. 07/245,183 describes anelectroconductive composition which comprises antimony-containing tinoxide in which the tin oxide is predominantly crystalline, and thecomposition exists in a unique association with silica or asilica-containing material, such as a silicate. The composition is apowder which can be dispersed in a liquid carrier and applied to asurface as a thin film and, when dried, will render the thin filmconductive.

A problem which has been encountered in practice with electroconductivepigments of the type comprising tiny particles of a substrate having anantimony-containing tin oxide conductive coating on each particle is inachieving a uniform dispersion of those particles in the carrier systemof choice and then maintaining the stability of the dispersion over aperiod of time, e.g., during storage. Stability can be particularlyimportant in connection with resin formulations for electroconductivepaint systems. It has been found that stable dispersions of paint resinswhich incorporate an antimony-containing tin oxide based conductivepigment can be achieved when the isoelectric point of the pigmentparticles has a value in the range of from about 5 to about 9. Theisoelectric point is significant because it represents the pH at whichthe surface of each particle has zero electrical charge, and, thereby,interactions of the individual particles with the resins of the paintsystem can be controlled.

SUMMARY OF THE INVENTION

The present invention is an electroconductive powder compositioncomprising powder particles which are generally smaller than 1000microns and frequently tens of microns to sub-micron in size having asurface coating layer of antimony-containing tin oxide which isconducting and an outer thin layer of a hydrous metal oxide having athickness of from a partial molecular layer to 5 monomolecular layers,i.e., from about 5 to 30 angstroms, and an isoelectric point in therange of from about 5 to 9. The present invention includes a method foradjusting the isoelectric point of such particles without substantiallyreducing their individual electroconductivity or their ability to forman interconnecting conductive network when dispersed in a carrier andapplied on a surface as a thin film by applying to the particles a thincoating layer of a high isoelectric point hydrous metal oxide.

The hydrous metal oxide contemplated for use in the invention is anessentially non-conducting oxide selected from the group consisting ofalumina, magnesia, zirconia, titania and rare earth metal oxides. Thehydrous metal oxide outer layer can be applied to theantimony-containing tin oxide layer by forming an aqueous slurry of thecoated particles and adding a solution of a salt containing the desiredmetal thereto while controlling the pH of the resulting solution witheither acid or base within the desired range of from about 5 to about 9.

Quite surprisingly, it is possible to adjust the isoelectric point ofelectroconductive particles whose conductivity, a predominantly surfaceproperty, is derived from a coating of antimony-containing tin oxide byapplying thereto a very thin layer of an essentially non-conductinghydrous metal oxide without substantially reducing the conductivity ofthe original particle.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1 and 2 are graphs which compare zeta potential over a pH range offrom 2 to 10 for aqueous dispersions of certain metal oxide-coatedelectroconductive particles according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is an electroconductive powder composition inwhich the individual particles which are smaller than 1000 microns andexhibit an isoelectric point in the range of from about 5 to 9 and whichis made possible from a thin outer coating of a generally non-conductinghydrous metal oxide, i.e., a coating layer having a thickness of fromabout 1 to about 5 monomolecular layers, i.e., from about 5 to 30angstroms. According to one embodiment of the invention, the coatinglayer can be as thin as a partial monomolecular layer.

The powder composition to which the invention is most applicablecomprises tens of microns to sub-micron size particles which are definedtypically by a generally nonconducting substrate, such as, for example,titanium dioxide or amorphous silica, with a conducting layer ofantimony-containing tin oxide. The particles which comprise the powdercomposition can be as large as 1000 microns, but they are generally notlarger than about 250 microns in size and usually smaller, i.e.,frequently tens of microns to sub-micron in size, although size is notcritical. U.S. Pat. No. 4,373,013, the teachings of which areincorporated herein by reference, describes the preparation of anelectroconductive powder in which titanium dioxide particles are used asnuclei, and their surfaces are coated with antimony-containing tinoxide. The shape of the particles comprising the powder is notparticularly critical to the invention, and the material used as thesubstrate is also not critical to the invention. For example, theparticles may be defined by an original core material having one or morecoating layers, the last of which is an antimony-containing tin oxideconducting layer. The particles may also be formed from a substratewhich is hollow, the preparation of which is described in greater detailin U.S. patent application Ser. No. 07/245,183, the teachings of whichare incorporated herein by reference.

Typically, electroconductive powders of the type described herein areused as pigments or additives in coating systems, such as for antistaticconductive paperboard. The paperboard, in turn, is used for packagingelectronic components and protecting them from latent or immediatefailure from electrostatic discharge. Another important use forelectroconductive powders is as a component of the pigment in automotivepaint primer compositions in which application of the paint top coat tothe automobile is by electrostatic spraying. A typical primercomposition will contain one or more solvents, resins, film formingbinders, cross-linking agents and/or various components of pigment. Byadjusting the isoelectric point of the particles to a value in the rangeof from 5 to 9 according to the invention, it is possible to reduce orcontrol the interaction of the particles with other components of theprimer and thereby improve the ability of the particles to remainuniformly dispersed within the composition. In paint systems where thepigment binders are catalyzed with acids or bases, for example, theisoelectric point of the pigment must be such as not to interferesignificantly with the curing process.

The isoelectric point is a useful measure of surface charge, and valuesof from 6 to 8, and preferably a value of 7, has been found to be verydesirable from the standpoint of paint formulation. By coating theelectroconductive pigment particles with a generally high isoelectricpoint hydrous oxide, which is practically non-conducting, it is possibleto preserve the electroconductivity of the particle while adjusting theisoelectric point of the particle from its typical range of from 1 to3-4 to a value within the desired range of from 5 to 9.

The outer hydrous metal oxide layer can be applied to theantimony-containing tin oxide coated particle by adding a solution ofthe salt which contains the desired metal to an aqueous slurry of theparticles to be coated while controlling the pH of the slurry witheither acid or base within the desired range. Metals contemplated foruse according to the invention are those whose hydrous oxides have highisoelectric points, i.e., in the range of from 5 to 9 or 10. Magnesiumand aluminum are examples of such metals; sodium aluminate is apreferred source of aluminum, and magnesium chloride is a preferredsource of magnesium. Other salts which can be used according to theinvention include chlorides, nitrates, and acetates. Where an acid saltis to be used as the source of metal, an alkali, such as sodiumhydroxide or potassium hydroxide, can be added to the aqueous slurry asappropriate to maintain the pH within the desired range. If a basic saltis to be used as the source of metal, an acid, such as hydrochloricacid, can be added to the slurry to maintain the pH.

The hydrous metal oxide used in the invention is selected from the groupconsisting of alumina, magnesia, zirconia, titania and rare earth metaloxides, with alumina and magnesia being preferred for reasons of ease ofapplication with readily available chemicals. When sodium aluminate isused, the pH of the resulting slurry is preferably maintained in therange of 7.0 to 9.0, and when magnesium chloride is used, the pH of theresulting slurry is preferably maintained at from 7.0 to 10.0. Coatingwith hydrous metal oxide can be accomplished at room temperature, butpreferably the temperature of the slurry during the coating processshould be maintained in the range of from 60° to about 90° C. forreasons of optimum rates and reproducibility.

The method of the invention can conveniently be carried out by:

(a) forming an aqueous slurry of electroconductive powder particles tobe coated in which the slurry has a resulting concentration of suchparticles in the range of from about 200 to 400 g/l and a temperature inthe range of from about 60° to about 90° C.;

(b) adding an aqueous solution of a salt containing a hydrous metaloxide having an isoelectric point in the range of from 6 to 10 to saidslurry while maintaining the pH of the resulting mixture in the range offrom about 5 to 9, and preferably from about 6 to 8;

(c) maintaining the mixture until from a partial to about 5monomolecular layers of metal oxide are deposited on the surface of saidparticles; and

(d) isolating the coated particles.

The thickness of the hydrous metal oxide coating is a function of saltconcentration added to the coating bath and concentration of conductivepowder particles in the bath. At a temperature of from 60° to 90° C. anda mixture pH in the range of from 7.0 to 10.0, the hydrous metal oxidewill ordinarily be deposited on the surface of the particles at a rateof about one monolayer/30 minutes.

The newly coated particles can be isolated by any convenientsolid-liquid separation procedure, such as, for example, by filtration,and then washed free of salts with water and dried. Drying is normallyaccomplished at temperatures up to about 120° C.

Referring now to the Figures, FIGS. 1 and 2 represent graphic plots ofZeta potential measurements on aqueous dispersions of electroconductiveparticles over a pH range of from 2 to 10. Curve "A" is a typical plotfor electroconductive particles comprising a hollow amorphous silicashell coated with a conducting layer of antimony-containing tin oxide.The isoelectric point, i.e., where the Zeta potential is zero, occurs atpH 2.5. For identical particles which have been coated with an outerlayer of hydrous aluminum oxide according to the invention, i.e., curve"B", the isoelectric point occurs at pH 7. FIG. 2 is a graphic plot ofZeta potential for the same silica shell particles, but having an outercoating layer according to the invention of magnesium oxide. Theisoelectric point for these magnesium-coated particles occurs at pH ofabout 9.

The coated electroconductive powders of this invention are characterizedby an enhanced isoelectric point which is typically in the range of fromabout 5 to 9, but preferably has a value of about 7. The isoelectricpoint for electroconductive particles based on antimony-containing tinoxide which are not coated according to the invention will typicallyrange from 1 to about 3.0, but they can even be less than zero.

Isoelectric point measurements were made using an automatedelectrokinetics analyzer known as Pen Ken System 3000 and manufacturedby Pen Kem, Inc., Bedford Hills, N.Y. The instrument measures theelectrophoretic mobility of particles in a dilute suspension.Measurements are made at different pH levels and by graphically plottingthe results, the isoelectric point, i.e., the pH at which Zeta potentialis zero, can be found. The invention can be further illustrated by thefollowing examples.

Ransburg units, i.e., surface conductivity, and dry powder resistivitieswere measured according to the method described in U.S. patentapplication Ser. No. 07/245,183, the teachings of which are incorporatedherein by reference.

EXAMPLE 1

An electroconductive powder was prepared according to the followingprocedure:

(A) In an 18-liter, agitated polyethylene beaker, 3 liters of water werebrought to a pH of 10.0 with NaOH. 100 g of potassium silicate (26.5%SiO₂) were added to form a solution. Thereafter, 1350 g of CaCO₃, whichhas previously been dispersed in 1 liter of water, were added. Theslurry was heated to 90° C. in one-half hour by the introduction ofsteam, after which the pH was 9.9. Next, 1027 g of potassium silicatesolution (26.5% SiO₂), predispersed in 1 liter of water, and 262 ml ofnominal 37% HCl, diluted to 1 liter with water, were addedsimultaneously to the slurry over a period of 5 hours. The pH wasmaintained at 9.0 during the addition of the two solutions. The slurrywas then digested at 90° C. for one-half hour, the pH was adjusted to7.0 with hydrochloric acid, and, after sedimentation, the supernatantwas decanted and the resulting mixture reheated to 90° C.

(B) Next, nominal 37% HCl was added until the pH reached 2.0. 1016 ml ofan aqueous SnCl₄ solution containing the equivalent of 0.286 g SnO₂ /ml,and 129 ml of an SbCl₃ /HCl solution, containing the equivalent of 0.235g Sb/ml, were combined and added to the slurry over a period of 2 hourssimultaneously with sufficient 30% NaOH to maintain the pH of the slurryat 2.0. The slurry was digested at a temperature of 90° C. for one-halfhour, and the resulting particles were filtered, washed with water toremove soluble salts, and then calcined at a temperature of 750° C. for2 hours. The finished powder product had a dry powder resistance of 25ohms. By X-ray fluorescence analysis, the powder was found to contain46% Sn (as SnO₂), 47% Si (as SiO₂), 6% Sb (as Sb₂ O₃) and 0.2% Ca (asCaO).

200 g of electroconductive powder prepared from the foregoingpreparation procedure were then added to one liter of deionized water ina round bottom flask equipped with a stirrer to form a slurry which wasthen heated to 65° C. While stirring continuously, 15 ml of sodiumaluminate solution containing 215 g Al₂ O₃ per liter, were addeddropwise over a period of one hour while the pH of the mixture wasmaintained between 8 and 8.5 by dropwise addition of 20% HCl. The sodiumaluminate solution contained 215 g Al₂ O₃ per liter. After addition ofthe sodium aluminate solution, the resulting mixture was stirred for anadditional 30 minutes at a pH of 8.5 and a temperature of 65° C. to curethe hydrous metal oxide coating.

The coated product was recovered by filtration using a Buchner vacuumfilter and washed with deionized water until free from soluble salts. Nochloride ion was detected in the final wash water passing through thefilter. The product was then air dried at 120° C. for 6 hours. It wasfound to contain 1.5% Al₂ O₃.

Isoelectric points (IEP) of the powders were determined using theautomated electrokinetic analyzer, Pen Ken System 3000, describedearlier. The IEP of the substrate, before applying the hydrous aluminacoating, was 2.4. The IEP of the coated electroconducting powder was7.5.

The coated powder had a dry electrical resistance of 100 ohms comparedwith 25 ohms for the powder prior to coating.

The powder was formulated with a test paint carrier at a pigment binderloading of 48/100 and applied to a test surface. The resulting dry paintfilm exhibited a surface conductivity of 160 Ransburg units, which wasslightly less than the value of 165 obtained for a formulation madeusing the powder prior to coating it with hydrous alumina.

Examples 2 and 3 were prepared as Example 1 with the difference that theamount of aluminate used was changed to give coatings corresponding to0.75% and 3.0% Al₂ O₃ based on the powder.

EXAMPLE 4

This Example illustrates the preparation of a powder consisting ofhollow shell amorphous silica particles coated with crystallites ofantimony-containing tin oxide to which a thin outer coating of hydrousmagnesium oxide has been applied. The outer coating corresponds to 1.0wt. % Mg(OH)_(x) based on the powder.

200 g of the substrate powder used in Example 1 were added to 1500 ml ofdeionized water in a round-bottom flask equipped with a stirrer,slurried and heated to 75° C. The pH was adjusted to 9.5 by the dropwiseaddition of 5% sodium hydroxide solution. Nine grams of MgCl₂.6H₂ O weredissolved in 50 ml of deionized water and this solution was addeddropwise to the slurry over a period of 2 hours, maintaining the pH at9.5 by the controlled addition of 5% sodium hydroxide solution. oncompletion of the addition of the magnesium chloride solution, theslurry was stirred for 30 minutes at a pH of 9.5 and a temperature to75° C. to cure the coating.

The powder was recovered and dried as described in Example 1 and wasfound to contain 1.0 wt. % Mg(OH)_(x). The isoelectric point of thesubstrate before applying the hydrous magnesia coating was 2.4, andafter applying the coating it was 8.8.

The coated powder had a dry powder electrical resistance of 180 ohmscompared with 25 ohms for the powder prior to coating. When formulatedwith a test paint carrier, as described in Example 1, the dry paint filmsurface conductivities were 140 and 165 Ransburg units, at 25 P/B and 48P/B (P/B=powder/binder ratio), respectively.

Table 1 summarizes the results obtained on Examples 1 through 4.

Electroconducting Powder #120 refers to the powder prepared according tosteps (A) and (B) of Example 1 and contains 46% Sn (as SnO₂), 47% Si (asSiO₂), 6% Sb (as Sb₂ O₃) and 0.2% Ca (as CaO).

                  TABLE 1                                                         ______________________________________                                                                         Conductive                                                                    Paint Film                                                         Powder Dry (Ransburg                                    Sample      Isoelectric                                                                             Resistance Readings)                                    Description Point     (Ohms)     48 P/B                                                                              25 P/B                                 ______________________________________                                        Electroconducting                                                                         2.4        25        165   160                                    Powder #120                                                                   Example 1   7.5       100        160   140                                    #120 + 1.5%                                                                   Al.sub.2 O.sub.3 Coating                                                      Example 2   6.7        40        165   155                                    #120 + 0.75%                                                                  Al.sub.2 O.sub.3 Coating                                                      Example 3   8.0       380        150   130                                    #120 + 3.0%                                                                   Al.sub.2 O.sub.3 Coating                                                      Example 4   8.8       180        165   140                                    #120 + 1.0%                                                                   Mg(OH).sub.x Coating                                                          ______________________________________                                    

I claim:
 1. An electroconductive powder composition comprising powderparticles smaller than 1000 microns comprising a nonconductingsubstrate, each such substrate having a first coating which comprises anetwork of interconnecting crystallites of antimony-containing tin oxideand a second outer coating layer of a hydrous metal oxide selected fromthe group consisting of alumina, magnesia, zirconia, titania and rareearth metal oxides and having a thickness of from about 1 monomolecularlayer to 5 monomolecular layers and an isoelectric point in the range offrom about 5 to 9.